1. Field of the Invention
The present invention relates generally to a magnetic rotational coupler, and more particularly to a separating pot for use in a magnetic centrifugal pump.
2. Description of the Related Art
Magnetic centrifugal pumps are utilized where an absolutely tight seal toward the outside is a concern since toxic, caustic or aggressive agents are to be pumped without escape into the environment. Such pumps preferably have suitable plastic linings for protection of the pump parts. Fluoroplastics are generally used as the plastic lining material in devices for use with highly aggressive agents.
A magnetic rotational coupler portion is provided in the magnetic centrifugal pump, the coupler portion including first and second magnetically coupled rotors separated by a separating diaphragm. A particular type of magnetic coupler has inner and outer rotors including magnets disposed in mutually coaxial cylinders for magnetic coupling between the rotors. A separating diaphragm in the shape of a pot is integrated between the force transmitting magnets of the inner and outer rotor members in such magnetic centrifugal pumps. The separating pot is to be also protected against attach by the toxic or caustic agent, at least at its inner surface.
The traditional separating pots are generally of one of three materials. In particular, the separating pot is either of solid plastic, of metal which includes a plastic insert or lining or which is enameled, or is of ceramic. All three materials, however, have severe disadvantages and limits when used in magnetic centrifugal pumps.
Solid plastic separating pots are limited to application in very low pressure and/or very low temperature stress situations. This type of separating pot can be used only within very limited operating conditions.
The use of metal separating pots having either a plastic or enamel protection results in the inducement of eddy currents in the metal due to the rotation of the inner and outer magnets. The eddy currents lead to losses in power transmission and to heating of the metal portions disposed between the magnets. Since the amount of heating is dependent upon the power transmitted, high power pumps can result in considerable heating. The insulating effect of a plastic layer over the metal reduces the amount of heat elimination possible through the agent being pumped. The temperature of the metal separating pot, thus, becomes so high that the plastic protective lining is deformed.
When an enameled metal separating pot is used, the heat generated in the metal is eliminated by conduction to the agent being pumped. This can cause the liquid to evaporate or cavitate, particularly when the agent to be pumped flows into the pump at elevated temperatures. Thus, the use of hot agents is limited in both cases. When the magnetic centrifugal pump is run in a dry state without cooling agents for the metal separating pot, inadmissible temperature limits are reached very quickly due to the eddy currents. Also, the power losses that occur when a metal pot is used must be compensated for by the provision of a larger motor and by higher current consumptions.
The use of a ceramic separating pot also has disadvantages. In particular, since ceramics are not well suited for withstanding tensile forces, a ceramic separating pot must be formed with very thick walls. This enlarges the spacing between the magnets of the inner and outer rotor and, thus, reduces the power that is transmitted thereby. Since the ceramic material is extremely sensitive to impact, the separating pot can be easily destroyed during a malfunction, such as a pressure surge or damage to a bearing. The destroyed separating pot can, thus, lead to escape of the toxic or caustic agent which was sought to be contained.